Modifies the translation keys for course statistics

fixes CNVS-15576

Test Plan:
  - Transifex should not fail on importing.

Change-Id: I70a9b453b1209389888e347e552ac9843c9acb40
Reviewed-on: https://gerrit.instructure.com/41200
Reviewed-by: Ryan Shaw <ryan@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Clay Diffrient <cdiffrient@instructure.com>